1. Field of the Invention
This invention relates, in general, to optical scanners and, more specifically, to illumination devices for producing reflected light from a scanned member.
2. Description of the Prior Art
Optical scanners can be used to convert optical indicia or information existing on a member or surface, such as a sheet of paper, into corresponding electrical signals. Such scanners usually require a means for illuminating the member being scanned. The reflected light from the member is then detected by a photosensitive device or imaged onto a sensor array, such as a charged coupled device (CCD), where the optical energy is converted into electrical signals or data. Laser diodes can be conveniently used for producing the illumination needed during this process. Laser diodes are desirable for this function since they are relatively inexpensive and power efficient. However, in some scanner applications the use of laser diodes is limited due to the limited light produced by a single diode.
In order to make laser diodes attractive alternatives in scanner illuminating applications which require high light intensities, the use of laser diode arrays has been suggested. These arrays use several diodes to increase the overall output power level to a workable value. U.S. Pat. No. 4,924,321, issued on May 8, 1990, discloses an image read-out device which uses a semiconductor laser array to provide the necessary illumination level during a read-out operation. The array uses a beam splitter and prisms to combine the separate laser beams into a single beam which is scanned or deflected across the member being scanned. While this approach may be beneficial in some cases, conventional beam splitters reduce the overall light output by approximately 50% since half of the beam energy passes through the silvered portion of the splitter and the other half is reflected. In the referenced patent, the number of diodes used during read-out and reproduction of an image is changed since the latter function of this device does not require the high illumination levels needed during the read-out mode.
Another important consideration in using the scanner illuminating system of the referenced patent is maintaining exact alignment of the spot of light produced by the laser beams, since reflected light from the entire spot is received by the sensor device. In other words, if the illuminating spot is displaced from the desired position the resulting electrical signal produced by the scanner can be in error. Therefore, it is desirable, and an object of this invention, to provide an illumination system which makes alignment less critical and which gives an efficient method of combining scanning light produced by a laser diode array.
Several characteristics of optical scanners can affect the uniformity and accuracy of scanned data obtained from the scanner. Such items as non-uniformity of an imaging lens or CCD sensor, either inherent in their design or because of manufacturing tolerances, scanned member location relative to the scanning spot, and scanning beam velocity changes across the scanned member can lead to inaccurate electrical signals representing the indicia on the scanned member. For these reasons, it is also desirable, and an object of this invention, to provide an efficient scanner system which can reduce or compensate for non-uniformities experienced by other scanner architectures.